I think that there is enough of difference between classes/jobs if you are dedicated to them (using them efficiently).
Some more variety would make things interesting for a short amount of time, then people would start saying the same thing again, because, people.
Putting 400 dps (nothing that special) on MNK and DRG is quite a different beast, especially when you forget about the class and start learning the fights (need to incorporate fight timings/mechanics into behavior).
Fights are the ones that dictate/limit/expand your play style. There are quite differences there (pressure on the mnk for GL3 for example and what he needs to do to keep it up).
You can't compare differences of the classes/jobs in normal dungeons because it just isn't needed.
If you haven't discover the specialization yet, that doesn't mean that it isn't there. And yes, its quite rewarding also (and if you don't sustain that specialization, you can lose it - efficiency at that class).